The present invention relates to a method for producing a semiconductor device having good electrical properties and also to a method for forming a thin film transistor (TFT) with good electrical properties.
Among thin film semiconductor elements, the TFT is well known. TFT is composed of an insulating substrate such as glass substrate and a thin film semiconductor (usually silicon semiconductor) as an active layer formed thereon which is several hundreds to several thousands of angstroms in thickness. TFT is applied to an electro-optical device such as a liquid crystal display device and an image sensor. Picture elements and peripheral drivers are formed by TFT formed directly on a glass substrate.
In using a glass substrate, the thin film silicon semiconductor formed on the substrate has amorphous or crystalline. A structure having crystalline represents the polycrystal structure, microcrystal structure, or a mixture of amorphous and crystalline structures. TFT based on an amorphous semiconductor is slightly poor in operating speed and electrical properties, and limits its application. By contrast, TFT based on a crystalline silicon film is capable of high speed operation and has good electrical properties.
TFT based on a crystalline silicon film has a problem associated with off state current. When a negative voltage is applied to the gate electrode in an N-channel type TFT, a current does not flow between a source and a drain in principle. This is because, by applying the negative voltage, a channel becomes a P-type and therefore a PN-junction is formed between the source and the drain. In actual, the crystalline silicon film contains crystal grain boundaries, crystal defects, and dangling bonds, so that a large number of levels produce. Accordingly, charges move in the reverse direction of PN-junction through these levels. When an electric field is concentrated at the PN-junction, a current leaks in the reverse direction through the defects and traps. as a result, an off current flows between the source and the drain by applying the negative voltage to the gate electrode.
A method to solve this problem is to form a lightly doped drain (LDD) region (lightly N-type), as an electric field relaxation region, between a channel (I-type) and a drain (N-type), which prevents the concentration of electric field between them.
Another method to obtain the same effect as the LDD region is to form an offset gate region which avoids the concentration of electric field between a channel and a drain. The offset gate region is a region which does not function as the drain between the channel and the drain.
As mentioned above, an off current of TFT decreases by defects and traps in the film. Also, defects and traps retard the movement of carriers in the film and therefore prevent the operation of TFT.
On the other hand, interface properties between a channel and a gate insulating film in TFT are extremely important. The interface properties greatly affect characteristics of TFT, and are evaluated in terms of interface level. The interface level produces by defects and dangling bonds. To obtain TFT having good characteristics, it is necessary to lower the interface level at the interface between the channel and the gate insulating film.
The object of the present invention is to solve the problems associated with an off current in a thin film transistor (TFT) and with an interface level at the interface between a channel and a gate insulating film. Therefore, The object of the present invention is to provide a method for lowering the level (related with dangling bonds) in thin film silicon semiconductor. Another object of the present invention is to provide TFT having good characteristics. Further another object of the present invention is to obtain a silicon semiconductor film having a less number of levels.
The present invention is embodied in a method for producing TFT comprising the steps of, forming a silicon semiconductor film on a substrate having an insulating surface, introducing into the silicon semiconductor film at least one of ionized and accelerated hydrogen, fluorine, and chlorine, and performing heat treatment on the silicon semiconductor film in an atmosphere containing one of hydrogen, fluorine, chlorine, and a mixture thereof.
The present invention is also embodied in a method for producing TFT comprising the steps of, forming an active layer, forming a gate insulating film on the active layer, forming a gate electrode on the gate insulating film, forming a silicon nitride film covering the gate insulating film and gate electrode, introducing hydrogen ions into the active layer through the gate insulating film and silicon nitride film, and performing heat treatment on the whole.
The substrate having an insulating surface includes a glass substrate, a glass substrate on which an insulating film is formed, a semiconductor substrate on which an insulating film is formed, a metal substrate on which an insulating film is formed, and other substrate made of an insulating material.
The silicon semiconductor film has amorphous silicon semiconductor and crystalline silicon semiconductor which are formed by plasma chemical vapor deposition (CVD) or low pressure heat CVD. An amorphous silicon film formed by CVD may be crystallized by heating or irradiation with a laser light or an equivalent intense light.
Hydrogen, fluorine, or chlorine ions may be implanted by using a well known ion implanting apparatus or a plasma doping apparatus. Ionization may be performed by producing plasma based on high frequency discharge or by mass separation. The ion implanting apparatus required in the present invention introduces, into the silicon semiconductor film, hydrogen, fluorine, or chlorine ions which an acceleration to voltage is applied.
When a plasma doping apparatus is used, it is possible to use hydrogen, chlorine, or fluorine as the doping gas. Hydrogen chloride, hydrogen fluoride, or the like can be used as the doping gas. In using hydrogen chloride as the doping gas, chlorine and hydrogen are introduced into the silicon semiconductor. A depth which each element is introduced varies depending on an ion and an acceleration voltage.
The purpose of heat treatment on the silicon semiconductor film in an atmosphere containing hydrogen, fluorine, chlorine, or a mixture thereof is to confine the introduced hydrogen atoms, fluorine atoms, or chlorine atoms in the silicon semiconductor and further to promote neutralization of dangling bonds caused by hydrogen, fluorine, or chlorine. The heat treatment can be performed in any atmosphere irrespective of the previously introduced element. For example, heat treatment in an atmosphere containing hydrogen or chlorine after chlorine implantation is permissible. Selection depends on the apparatus to be used and the desired effect.
The atmosphere for heat treatment is not limited to that of single element; it may be a mixture of gases or a gaseous compound. An atmosphere containing a hydrogen-nitrogen mixture having a desired ratio or hydrogen chloride can be used.
The selection of hydrogen, chlorine, or fluorine for implantation depends on the apparatus employed and the characteristic required of the silicon semiconductor. In general, hydrogen implantation is easy. Deep implantation is possible with hydrogen (which is light in weight) at a low acceleration voltage. This almost avoids damages to the silicon semiconductor.
Deep implantation of fluorine or chlorine (which has larger ionic radius) needs a high acceleration voltage, which causes appreciable damages to the silicon semiconductor. However, these elements are not easily released by an external electric field because of their large ionic radius and their high bond energy for silicon. Chlorine, fluorine, and hydrogen decrease in bond energy for silicon in the order listed. The chlorine-silicon bond energy does not differ greatly from the fluorine-silicon bond energy; however, the hydrogen-silicon bond energy is extremely small. For example, the chlorine-silicon bond and fluorine-silicon bond begin to dissociate at 500xc2x0 C., whereas the hydrogen-silicon bond begins to dissociate at 150-200xc2x0 C. and completely decomposes at 350-500xc2x0 C.
Implantation of hydrogen ions through the silicon nitride film which has previously been formed is intended to confine the introduced hydrogen ions in the active layer and to maintain their effect. That is, the silicon nitride film prevents the release of hydrogen, thereby enhancing the effect of hydrogen ion implantation and stabilizing the device.
The silicon nitride film functions as a barrier layer for the introduced hydrogen ions, viz, it prevents hydrogen from gasifying and releasing itself. An aluminum nitride film, aluminum oxide film, aluminum oxide nitride (AlOxNy) film, or silicon oxide nitride (SiOxNy) film is used as a barrier layer. Silicon oxide nitride film functions effectively also as a film to cover the device because of its ability to relax stress more than silicon nitride film.
The heat treatment performed on the whole is intended to confine the introduced hydrogen atoms in the silicon semiconductor and to neutralize dangling bonds with hydrogen. The heat treatment may be performed in an atmosphere containing H2, N2, Ar, He, or O2. By this heat treatment, dangling bonds in the silicon film are neutralized with introduced hydrogen ions and hence the levels (traps) and defects due to dangling bonds are reduced.
The present Invention is also embodied in a method for producing TFT comprising the steps of, forming a silicon semiconductor film on a substrate having an insulating surface, forming an insulating film on the silicon semiconductor film, and introducing at least one of ionized hydrogen, fluorine, and chlorine through the insulating film. The last step may be followed by heat treatment in an atmosphere containing hydrogen, fluorine, or chlorine, or a mixture thereof.
The present invention is also embodied in a method for producing TFT comprising the steps of, forming an active layer, forming a gate insulating film on the active layer, introducing hydrogen ions into the active layer through the gate insulating film, forming a gate electrode on the gate insulating film, forming a silicon nitride film covering the gate insulating film and gate electrode, introducing hydrogen ions into the active layer through the gate insulating film and silicon nitride film, and performing heat treatment on the whole.
In general, TFT that employs a silicon semiconductor film takes the structure of insulated gate type field effect transistor, with the gate insulating film being silicon oxide film or silicon nitride film. In this case, the characteristic of the interface between the silicon semiconductor film and the gate insulating film is extremely important. The implantation of hydrogen, chlorine, or fluorine ions, with the insulating film formed on the silicon semiconductor film, neutralizes dangling bonds of silicon in the silicon semiconductor and reduces an interface level at the interface between the silicon semiconductor and the insulating film. Since the interface level is due to dangling bonds, the implantation of hydrogen, fluorine, or chlorine neutralizes dangling bonds and reduces interface levels. Whether to implant hydrogen, chlorine, and fluorine individually or in combination depends on the apparatus employed and the characteristic desired.
The ion implantation can be performed such that the projected range of hydrogen, fluorine, and chlorine ions is in the neighborhood of the interface between the silicon semiconductor film and the insulating film. The projected range is an index representing a depth having the highest probability with respect to positions of ions introduced in the solid. Defining the projected range as above means that there exist hydrogen, chlorine, or fluorine mostly in the vicinity of the interface between the silicon semiconductor film and the insulating film.
FIGS. 18 and 19 show depth profiles of hydrogen ion concentration by SIMS. These depth profiles relate to a silicon wafer into which hydrogen ions are introduced. In FIG. 8, an acceleration voltage is 20 kV and a dose is 1xc3x971016 cmxe2x88x922. In FIG. 19, an acceleration voltage is 40 kV and a dose is 1xc3x971016 cmxe2x88x922. A thickness of the silicon wafer is about 500 xcexcm or more. As can be seen from FIGS. 18 and 19, there are peaks representing a maximum ion concentration at desired depths. The depths are about 0.12 xcexcm (FIG. 18) and about 0.22 xcexcm (FIG. 19). Therefore, a depth of the peak changes in accordance with an acceleration voltage.
In FIG. 18, a hydrogen ion concentration in the depth of 0.12 xcexcm is about 2xc3x971021 atoms/cm3. Also, from FIG. 19, a minimum hydrogen ion concentration is about 5xc3x971017 atoms/cm3. In general, a concentration of single crystalline silicon is about 5xc3x971022 cmxe2x88x923. As a result, a concentration of hydrogen ions introduced into the silicon wafer is within a range of about 0.001 to 5 atoms %.
Note that a depth profile of hydrogen ions introduced into an insulating film such as a silicon oxide film almost coincide with the above depth profiles with respect to the silicon wafer.
Therefore, since neutralization of dangling bonds in silicon is performed mainly in the vicinity of the interface between the silicon semiconductor film and the insulating film, it is possible to greatly reduce a level at the interface between the silicon semiconductor film and the insulating film. Since hydrogen, chlorine, and fluorine differ in mass, some elements would be present near the interface but other elements would not, if a plurality of elements are introduced. Which element should be present near the interface depends on the characteristic desired. In general, the interface characteristic would be stable if chlorine and fluorine are present near the interface, because the bond energy between chlorine and silicon or between fluorine and silicon is greater than that between hydrogen and silicon.
It is desirable that the projected range for hydrogen, fluorine, and chlorine ions be slightly shifted toward the silicon semiconductor film from the interface between the silicon semiconductor film and the insulating film. This is because dangling bonds in silicon (which are responsible for interface levels) are present more in the silicon semiconductor film.
After element implantation, when heat treatment is performed a desired temperature in an atmosphere containing hydrogen, fluorine, chlorine, or a mixture thereof, the hydrogen that neutralizes dangling bonds is confined. In other words, as the result of heat treatment, these elements terminate the dangling bonds due to ion implantation into the silicon semiconductor and the insulating film formed thereon, to obtain a stabler state. The atmosphere for heat treatment is independent of the element which has been introduced. It is not limited to that of single element; it may be a mixture of gases or a gaseous compound.
The present invention is also embodied in a method for producing TFT comprising the steps of, introducing an impurity for providing one conductivity type into the active layer using the gate electrode as a mask, to form the source and drain regions, and introducing at least one of hydrogen, fluorine, and chlorine into the source and drain regions.
FIGS. 3A to 3E show an example of the process which employs hydrogen for implantation. In FIG. 3B, phosphorus ions (P+) are introduced by using as a mask the gate electrode 14 and its surrounding oxide layer 15, so that the source region 16 and the drain region 18 are formed. Then, hydrogen ions are implanted. The implantation of hydrogen ions may be performed before the implantation of phosphorus ions. The region (or junction portion) where the conductivity type of impurity changes is often subject to an application of an intense electric field. Such junction portions include PN-junction, PI-junction, and NI-junction. When there exist dangling bonds in the silicon semiconductor, in the insulating film, or in their interface, near the junction, electrons and holes released (emitted) by the intense electric field are captured by such dangling bonds. They function as the charge center and induce the semiconductor characteristic of N-type or P-type conductivity in the substantially intrinsic (I-type) semiconductor region.
Such dangling bonds can be terminated by hydrogen, chlorine, or fluorine. The effective introduction of these elements may be performed by ion implantation. Whether to use hydrogen, chlorine, and fluorine alone or in combination depends on the apparatus employed or the characteristic desired. After the element implantation, when heat treatment is performed at a desired temperature in an atmosphere containing hydrogen, fluorine, chlorine, or a mixture thereof, the heat treatment completes the hydrogenation and minimizes a level due to dangling bonds in silicon. That is, these elements terminate dangling bonds due to ion implantation into the silicon semiconductor and the insulating film formed thereon. This leads to a stabler state. The heat treatment may be performed in an atmosphere containing any element irrespective of the element used for the ion implantation. The atmosphere is not limited to that of single element; it may be a mixture of gases or a gaseous compound.
The present invention is embodied in a method for producing TFT comprising the steps of, introducing an impurity for providing one conductivity type into the active layer using the gate electrode as a mask to form the source and drain regions, irradiating the source and drain regions with a laser beam or an equivalent intense light to perform annealing, and introducing at least one species of hydrogen, fluorine, and chlorine into the source and drain regions.
FIGS. 4A to 4E show an example of the process which employs hydrogen for implantation. In FIG. 4B, phosphorus ions are introduced by using the gate electrode 14 and its surrounding oxide layer 15 as masks. In FIG. 4C, a laser light is irradiated to anneal the damage caused by the previous ion implantation and to activate the implanted phosphorus ions. Hydrogen which terminates dangling bonds in the silicon semiconductor releases from the silicon semiconductor. If the laser irradiation eliminates dangling bonds completely, there is no problem. However, since an irradiation time of a pulse laser is short, the laser irradiation tends to give great strains to the crystals. Accordingly, hydrogen ion implantation of FIG. 4D replenishs the hydrogen released from the silicon film as the results of ion implantation of FIG. 4B and laser irradiation of FIG. 4C. When heat treatment is performed on the whole in an atmosphere containing hydrogen as shown in FIG. 4E, hydrogen is confined in the silicon film to make the hydrogen ion implantation more effective.
In the present invention, the hydrogen ion implantation is performed through the layer that functions as a barrier layer, and then heat treatment is performed. Also, in the present invention, hydrogen ions are introduced into the active layer of silicon semiconductor film using the gate electrode as a mask. In plasma doping, H+ ions, H2+ions, and H3+ions are introduced; however, the gate electrode blocks H2+ ions and H3+ ions having a large ionic radius. That is, the gate electrode functions as a mask for H2+ ions and H3+ ions.
The implantation of hydrogen, fluorine, or chlorine ions into the silicon semiconductor film, the insulating film, or the interface between them neutralizes dangling bonds in the silicon film and improves the electrical properties of the silicon film. Unlike the ordinary heat treatment in an atmosphere containing hydrogen, chlorine, or fluorine, the ion implantation permits the effective introduction of these elements into the silicon semiconductor film, the insulating film, or the interface between them. With the ordinary heating treatment, it is very difficult to penetrate chlorine or fluorine into crystalline silicon; this disadvantage is eliminated in this invention by implantation of electrically accelerated ions.
Implantation of hydrogen, fluorine, or chlorine ions into the silicon film with an insulating film formed thereon reduces a level at the interface between the silicon film and the insulating film. It is desirable that the projected range for hydrogen, fluorine, and chlorine ions be close to the interface between the silicon film and the insulating film. When a hydrogen ion is implanted after formation of the silicon nitride, and then heat treatment is performed in an atmosphere containing hydrogen, hydrogen atoms can be confined in the silicon film (owing to the action of the silicon nitride film), and the trap level and defects due to hydrogen can be effectively reduced.
When the ion implantation of impurity into the silicon film and the irradiation with a laser light are followed by the implantation of hydrogen, fluorine, or chlorine ions into the silicon film, these processes replenish the hydrogen (dangling bonds and interface levels which are produced in the silicon film, the insulating film, or the interface between them) released by the ion implantation and the laser irradiation.
In all the cases, hydrogen, chlorine, or fluorine ions is implanted and then heat treatment is performed in an atmosphere containing hydrogen, fluorine, or chlorine, or a mixture thereof. This heat treatment confines the introduced ions in the silicon film, to enhance the effect.